In such a known servo drive (U.S. Pat. No. 3,878,764), in which of two servomotors only one operates on a common output and the other one at such time follows pressureless in standby condition, a positive, but force limiting, connection is provided between each control valve piston rod and the member which relays control signals to it. This connection permits in case of malfunctioning of one control valve sleeve valve an "overriding" of same and the second and presumably operative control valve sleeve valve (of the second servomotor) can still be moved by means of the member which transmits the relevant control signals. The changeover device is advantageously in active engagement with the connection which exists between the control valve piston rod of the (first) servomotor, which serves as a main drive, and the member, which relays control signals to this piston rod so that a blocking of the sleeve valve in the control valve of the first servomotor effects an automatic switching over to the second and presumably operative servomotor or its control valve.
In this double-hydraulic actuator, the members which relay the control signals are, for example, levers which are coupled through a rod which is hinged thereon, and are pivotally hinged to the operating piston rod of the respectively associated servomotor. The positive but force limiting connection between each lever and the associated control valve piston rod, thus permits a further pivoting of the lever when the sleeve valve of the respective control valve blocks. Thus a blocked control valve sleeve valve does not result in a blocking of the entire linkage. Only control forces -- exceeding the normal control forces -- must be produced for the further pivoting of the levers or the "overriding" of the blocked control valve sleeve valve. An "overriding" due to an overloading of the positive connection can, however, occur also when same is loaded in pressureless condition of the actuator through adjustment of the linkage or pivoting of the levers, for example during servicing of the system which is connected to the actuator. The operating piston rods of the actuator are often in this case moved directly through the linkage of the levers, in order to effect a control in the connected system. For this purpose, positive control forces must be produced, which considerably exceed the normal control forces of a hydraulically active actuator. The consequence is an undesired frequent stress and thus a wear of the connections which inevitably eventually creates damage. The latter is particularly true for members which are in direct active engagement with the positive connections, like microswitches and the like. Furthermore, there exists also a problem in the controlling of the "overridability" of the sleeve valves at a pressureless actuator. If such an "overriding" is by mistake and not corrected, it can lead to serious consequences during operation.